Use inner class as collection

Is there a way to get toplink to use an inner class collection to house my 1:m relationship. The collection needs some attributes of the containing object.
Can i override how collection is constructed within the java, because it seems pretty clear the workbench doesn't support this.
thanks
craig

Seem like an odd thing to do. If it were a static inner class it would be possible, but an instance inner class is probably not.
If you really need a reference to the source object from the collection object, it might be better to accomplish this through making the inner collection class an outer class, or at least static. You could then set a back reference to the source object in the source object's set method for the collection. You would need to configure the mapping to use method access, or if using value holders you could lazily set the collection back reference from your get method for the collection. You could also make special get/set methods for TopLink to use to access the collection value that convert the special collect to and from a normal collection.
In general it might be better to put the logic specific to the owning class in the owning class instead of the collection.

Similar Messages

When will we use inner class

hi
when will we use inner class?

JosAH basically already wrote this, but another way
to say the same thing is this. If you are writing a
class and you find it would be handy to have another
class for that class to use, you can either write a
separate class for each, or you can embed one in the
other, making it an inner class. One advantage of an
inner class is that you can meet a set of
specifications by not having an extra class. It also
makes more sense to have an inner class if it's
something that's not useful on its own.I think it is the second most important advantage of inner class
(First is: you can access the private members of the outer class)

Design question about when to use inner classes for models

This is a general design question about when to use inner classes or separate classes when dealing with table models and such. Typically I'd want to have everything related to a table within one classes, but looking at some tutorials that teach how to add a button to a table I'm finding that you have to implement quite a sophisticated tablemodel which, if nothing else, is somewhat unweildy to put as an inner class.
The tutorial I'm following in particular is this one:
http://www.devx.com/getHelpOn/10MinuteSolution/20425
I was just wondering if somebody can give me their personal opinion as to when they would place that abstracttablemodel into a separate class and when they would just have it as an inner class. I guess re-usability is one consideration, but just wanted to get some good design suggestions.

It's funny that you mention that because I was comparing how the example I linked to above creates a usable button in the table and how you implemented it in another thread where you used a ButtonColumn object. I was trying to compare both implementations, but being a newbie at this, they seemed entirely different from each other. The way I understand it with the example above is that it creates a TableRenderer which should be able to render any component object, then it sets the defaultRenderer to the default and JButton.Class' renderer to that custom renderer. I don't totally understand your design in the thread
http://forum.java.sun.com/thread.jspa?forumID=57&threadID=680674
quite yet, but it's implemented in quite a bit different way. Like I was saying the buttonClass that you created seem to be creating an object of which function I don't quite see. It looks more like a method, but I'm still trying to see how you did it, since it obviously worked.
Man adding a button to a table is much more difficult than I imagined.
Message was edited by:
deadseasquirrels

Why and how to use "inner class"

When i am learning advanced java language features,
i could not understand why and when to use the "inner class",
who can give me some examples?
Thanks!

You would use an inner class when an object needs visibility of the outer class. This is akin to a C++ friend.
An example of this is an iterator. An iterator over some collection is typically implemented as an inner class of the collection class. The API user asks for an Iterator (from the Collection) and gets one - in fact they receive an instance of an inner class, but doesn't care. The iterator needs to be inner, as the iterator needs to see the internal data structures of the outer (collection) class.
This could also be done with an anonymous class, as mentioned by spenhoet above. However, in the case of a collection, the role of the iterator is clear - thus it deserves its own class. And often there is more than one place an iterator can be returned (e.g. see java.util.List, which has several methods that return Iterators/ListIterators), thus it must be put in its own class to allow reuse of the code by outer class.

Help: Factory Class using Inner Class and Private Constructor?

The situation is as follows:
I want a GamesCollection class that instantiates Game objects by looking up the information needed from a database. I would like to use Game outside of GamesCollection, but only have it instantiated by GamesCollection to ensure the game actually exist. Each Game object is linked to a database record. If a Game object exist, it must also exist in the database. Game objects can never be removed from the database.
I thought about making the Game object an inner class of GamesCollection, but this means that Game class constructor is still visible outside. So what if I made Game constructor private? Well, now I can't create Game objects without a static method inside Game class (static Object factory).
Basically what I need is a constructor for the inner Game class accessible to GamesCollection, but not to the rest of the world (including packages). Is there a way to do this?

leesiulung wrote:
As a second look, I was initially confused about your first implementation, but it now makes more sense.
Let me make sure I understand this:
- the interface is needed to make the class accessible outside the outer classBetter: it is necessary to have a type that is accessible outside of GameCollection -- what else could be the return type of instance?
- the instance() method is the object factory
- the private modifier for the inner class is to prevent outside classes to instantiate this objectRight.
However, is a private inner class accessible in the outer class? Try it and see.
How does this affect private/public modifiers on inner classes?Take about five minutes and write a few tests. That should answer any questions you may have.
How do instantiate a GameImpl object? This basically goes back to the first question.Filling out the initial solution:
public interface Game {
    String method();
public class GameCollection {
    private static class GameImpl implements Game {
        public String method() {
            return "GameImpl";
    public Game instance() {
        return new GameImpl();
    public static void main(String[] args) {
        GameCollection app = new GameCollection();
        Game game = app.instance();
        System.out.println(game.method());
}Even if you were not interested in controlling game creation, defining interfaces for key concepts like Game is always going to be a good idea. Consider how you will write testing code, for example. How will you mock Game?

Help,about why we use inner class?

Hi,
when i read "java Tutorial"
i found there is one chapter about inner class .
i copy it down as follow.
the context is about there is a class Stack, and this class want to implement some function of interface Iterator,but as the book said
we should not let class Stack implement the Iterator directly, we should add a inner class inside the Stack .
i know it's very import ,but i still can not understand the reason why add a inner class here.
hope somebody can explain it a little more for me or give an example.
thank in advance!
Iterator defines the interface for stepping once through the elements within an ordered set in order. You use it like this:
while (hasNext()) {
next();
The Stack class itself should not implement the Iterator interface, because of certain limitations imposed by the API of the Iterator interface: two separate objects could not enumerate the items in the Stack concurrently, because there's no way of knowing who's calling the next method; the enumeration could not be restarted, because the Iterator interface doesn't have methods to support that; and the enumeration could be invoked only once, because the Iterator interface doesn't have methods for going back to the beginning. Instead, a helper class should do the work for Stack.
The helper class must have access to the Stack's elements and also must be able to access them directly because the Stack's public interface supports only LIFO access. This is where inner classes come in.
Here's a Stack implementation that defines a helper class, called StackIterator, for enumerating the stack's elements:
public class Stack {
private Object[] items;
//code for Stack's methods and constructors
not shown
public Iterator iterator() {
return new StackIterator();
class StackIterator implements Iterator {
int currentItem = items.size() - 1;
public boolean hasNext() {
public Object next() {
public void remove() {
or you can visit here
http://java.sun.com/docs/books/tutorial/java/javaOO/innerclasses.html

the context is about there is a class Stack, and this
class want to implement some function of interface
Iterator,but as the book said
we should not let class Stack implement the Iterator
directly, we should add a inner class inside the
Stack .Simply because the implementation of the Iterator is nobody's business. By declaring it to be a private inner clss, nobody will ever know about it and only see the Iterator interface.

Help! - JRE can't find classes that use inner classes!

Whenever I try to launch an app using the JRE, if the class was built with an inner class definition, the JRE says it can't find the class.
For instance:
I created a class called GUI, which contains anonymous inner class definitions for the button handlers: addActionListener(new ActionListener()){, etc.
When I compile this class I get GUI.class and GUI$1.class
When I try to launch this using the JRE:
jre -cp c:\progra~1\myApps GUI
The JRE spits out
'Class not found: GUI'
BUT if I go back and remove the action handlers so that the only compiled class file is GUI.class (not GUI$1.class) then the JRE can find the class no problem.
What in the good Lord's name is going on here?

Thanks for the response. Got the 1.2.2 version of the JRE, where I guess you no longer invoke jre.exe (no such file exists) but java.exe instead (not well documented).
Also, the newer version of the JRE has no problems locating classes, so I guess I'm OK.

How to use inner class in axis2 in java?

Hi,
Iam very new in axis2. My language is java. What iam trying to do is, i need to create a xml format for the clients to send data to server using axis2 service.
My demo xml format is,
<test1>
<test2>
<test3>
<test4></test4>
<test5></test5>
</test3>
<test3>
<test4></test4>
<test5></test5>
</test3>
</test2>
</test1>
Iam trying to use nested inner class to generate this xml. Below is my demo java class. But unfortunately, or may be lac of knowledge, iam unable to create sub trees.
public class EchoService{
public MyInnerClass retMyInnerClass(MyInnerClass test1) {
return test1;
public class MyInnerClass {
private String test2;
public MyInnerClass() { }
public void setTest2(String test2) {
this.test2 = test2;
public String getTest2() {
return this.test2;
Please help me to generate the web service for client.
Thanks in advance.

847897 wrote:
well, i apologies for that fault. But i need the answer. It is urgent.That's your problem, not ours. However, I don't see the connection between your inner class and producing XML subtrees. Perhaps if you explained the problem, rather than your solution...
[url http://sscce.org/]This page is usually a good place to start; otherwise [url http://www.catb.org/~esr/faqs/smart-questions.html]this one.
Winston

Please give me the information when to use inner classes in applications

HI,
please give me when to use the innerclasses

> HI,
please give me when to use the innerclasses
Use them when you want to create a class within another class.
Now gimme all 'er Dukes!!!

Inner classes use

what is the main goal behind using inner classes in java design

The best thing about inner classes in Java (something they missed in the C++ spec) is that a non-static inner class has direct access to everything in the containing class without the need to maintain an explicit reference.
That makes inner class instances ideal as a kind of "delegate" from the main object into another context, e.g. to generate several ActionListener objects to be added to various gadgets, or as an Iterator which moves through some child elements.

USE OF INNER CLASS

Dear All
PLease Expalin Why do we USE INNER CLASS What are the benefits of it.
Help is Greatly Appreciated.

Check out http://java.sun.com/docs/books/tutorial/java/javaOO/innerclasses.html

Problem with final variables and inner classes (JDK1.1.8)

When using JDK1.1.8, I came up with following:
public class Outer
    protected final int i;
    protected Inner inner = null;
    public Outer(int value)
        i = value;
        inner = new Inner();
        inner.foo();
    protected class Inner
        public void foo()
            System.out.println(i);
}causing this:
Outer.java:6: Blank final variable 'i' may not have been initialized. It must be assigned a value in an initializer, or in every constructor.
public Outer(int value)
^
1 error
With JDK 1.3 this works just fine, as it does with 1.1.8 if
1) I don't use inner class, or
2) I assign the value in initializer, or
3) I leave the keyword final away.
and none of these is actually an option for me, neither using a newer JDK, if only there is another way to solve this.
Reasons why I am trying to do this:
1) I can't use a newer JDK
2) I want to be able to assign the variables value in constructor
3) I want to prevent anyone (including myself ;)) from changing the value in other parts of the class (yes, the code above is just to give you the idea, not the whole code)
4) I must be able to use inner classes
So, does anyone have a suggestion how to solve this problem of mine? Or can someone say that this is a JDK 1.1.8 feature, and that I just have to live with it? In that case, sticking to solution 3 is probably the best alternative here, at least for me (and hope that no-one will change the variables value). Or is it crappy planning..?

You cannot use a final field if you do not
initialize it at the time of declaration. So yes,
your design is invalid.Sorry if I am being a bit too stubborn or something. :) I am just honestly a bit puzzled, since... If I cannot use a final field in an aforementioned situation, why does following work? (JDK 1.3.1 on Linux)
public class Outer {
        protected final String str;
        public Outer(String paramStr) {
                str = paramStr;
                Inner in = new Inner();
                in.foo();
        public void foo() {
                System.out.println("Outer.foo(): " + str);
        public static void main( String args[] ) {
                String param = new String("This is test.");
                Outer outer = new Outer(param);
                outer.foo();
        protected class Inner {
                public void foo() {
                        System.out.println("Inner.foo(): " + str);
} producing the following:
[1:39] % javac Outer.java
[1:39] % java Outer
Inner.foo(): This is test.
Outer.foo(): This is test.
Is this then an "undocumented feature", working even though it shouldn't work?
However, I assume you could
get by with eliminating the final field and simply
passing the value directly to the Inner class's
constructor. if not, you'll have to rethink larger
aspects of your design.I guess this is the way it must be done.
Jussi

Java Inner classes within the Threading paradigm

Hi all.
Im familiar with static nested classes, method local classes, inner classes and anonymous classes.
But what I am a little perplexed by, is when exactly to use inner classes or static nested classes? (forgetting anonymous classes and method local classes).
I read this article (http://www.javaworld.com/javaworld/javaqa/2000-03/02-qa-innerclass.html) and the first point makes for a good argument, but why nest the class? Why not define it as an external class?
Also you typically find nested classes within the Threading paradigm, but why?
I typically would create a top level class (non nested) that would implement the Runnable interface, override run() and then
use this class when constructing a new Thread.
Any clarification would be greatly received.
Thanks and Happy Friday.

Boeing-737 wrote:
I read this article (http://www.javaworld.com/javaworld/javaqa/2000-03/02-qa-innerclass.html) and the first point makes for a good argument, but why nest the class? Why not define it as an external class?
I was going to provide some reasons, but when I read the article I found they were already there. Following the DRY principle I'm going to let the article speak for itself.
Also you typically find nested classes within the Threading paradigm, but why?No, I don't. If that's where you typically find them then you haven't had a varied experience.
I typically would create a top level class (non nested) that would implement the Runnable interface, override run() and then
use this class when constructing a new Thread.So carry on doing that, if it works for you.

PaintComponent: inner class / method?

Hi,
I am doing a lot of painting in my paintComponent method and would like to ask whether it's possible to use e.g. inner classes or methods within paintComponent? I think I am confronted with the problem that my paintComponent method gets too unstructured :-/
Thanks for your help!

I am doing a lot of painting in my paintComponent
method and would like to ask whether it's possible to
use e.g. inner classes or methods within
paintComponent? I think I am confronted with the
problem that my paintComponent method gets too
unstructured :-/paintComponent is a method... so, you can call other methods from it. I'm not sure what you mean by "use inner classes". You can define an inner class and instantiate an object of that class from within paintComponent just like you can from any other method. Your question seems a bit weird, or I'm not understanding it.
Is your paintComponent too slow or just too long? By all means, use methods to logically separate different things you are doing, but this only add organization, not speed.

General class and setting properties vs specific inner class

This is a general question for discussion. I am curious to know how people differentiate between instantiating a standard class and making property adjustments within a method versus defining a new inner class with the property adjustments defined within.
For example, when laying out a screen do you:
- instantiate all the objects, set properties, and define the layout all within a method
- create an inner class that defines the details of the layout (may reference other inner classes if complex) that is then just instantiated within a method
- use some combination of the two depending on size and complexity.
- use some other strategy
Obviously, by breaking the work up into smaller classes you are simplifying the structure since each class is taking on less responsibility, as well as hiding the details of the implementaion from higher level classes. On the other hand, if you are just instantiating an object and making some SET calls is creating an inner class overkill.
Is there a general consensus for an approach? I am curious to hear the approach of others.

it's depends on your design..
usually, if the application is simple and is not expected to be maintain (update..etc..) than I just have all the building of the gui within the same class (usually..the main class that extends JFrame).
if the application follows the MVC pattern, than I would have a seperate class that build the GUI for a particular View. I would create another class to handle the ActionEvent, and other event (Controller)
I rarely use inner class...and only use them to implements the Listerner interface (but only for simple application)..

Use inner class as collection

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